Understanding the mechanisms that control vertebrate head development is an important problem since craniofacial anomalies account for over one third of all birth defects. Treacher Collins Syndrome (TCS) is a craniofacial disorder affecting approximately 1 in 50000 live births. Mutations in TCOF1 are well known to underlie the pathogenesis of TCS through disrupting the development of a migratory stem and progenitor cell population called neural crest (NC) cells1. Recently, mutations in two new genes POLR1Cand POLR1D were also identified in patients with TCS2. POLR1C and POLR1D encode subunits of RNA polymerase I and III, but nothing is known about the role of these genes in NC cell and craniofacial development. As most craniofacial anomalies are largely attributed to defects in the formation, proliferation, migration, and/or differentiation of NC cell, it is hypothesized that mutations in POLR1C and POLR1D may disrupt ribosome biogenesis in NC cells and contribute to the pathogenesis of TCS. This work will examine how disruptions in global processes such as ribosomal RNA transcription and ribosome biogenesis can elicit very specific congenital defects. Results from these studies will provide new models for studying TCS and new information on the roles of polr1c and polr1d in NC and craniofacial development. In Aim 1 of this proposal, we will determine the role of polr1c and polr1d during embryonic development in order to investigate the mechanisms underlying the pathogenesis of TCS and identify new avenues for prevention. We hypothesize Polr1c and Polr1d function dynamically during embryogenesis which we will demonstrate via in situ hybridization for polr1c and polr1d. In addition, we hypothesize that polr1c and polr1d loss of function will perturb ribosome biogenesis leading to defects in NC cell formation, proliferation, migration, or differentiation. I the second aim of this proposal, we will investigate two avenues for prevention of the craniofacial malformations associated with TCS. First, we will demonstrate through TUNEL and Western blot analysis that p53-dependent apoptosis contributes to craniofacial anomalies in polr1c and polr1d mutant zebrafish. We hypothesize that genetic inhibition of p53 in polr1c and polr1d zebrafish will prevent the pathogenesis of craniofacial anomalies. As a second avenue for prevention, we hypothesize that nutritional stimulation of ribosome biogenesis can prevent the craniofacial anomalies in polr1c and polr1d mutant zebrafish. We will determine the mechanisms of rescue and hypothesize that p53 inhibition will prevent the loss of neuroepithelial cells and restore NC formation, migration, and differentiation. We hypothesize that dietary leucine will stimulate ribosome biogenesis and proliferation while reducing neuroepithelial cell death, restoring the NC cell population. Collectively, this will provide new information on the mechanisms by which mutations in polr1c and polr1d result in craniofacial malformations. Additionally, these aims will identify new therapies for the treatment of TCS in humans which could alleviate the need for extensive corrective surgeries.